text: The efficiency of a conventional steam-electric power plant, defined as energy produced by the plant divided by the heating value of the fuel consumed by it, is typically 33 to 48%, limited as all heat engines are by the laws of thermodynamics (See: Carnot cycle). The rest of the energy must leave the plant in the form of heat. This waste heat can be removed by cooling water or in cooling towers. (cogeneration uses the waste heat for district heating). An important class of steam power plants is associated with desalination facilities, which are typically found in desert countries with large supplies of natural gas. In these plants freshwater and electricity are equally important products.
  Since the efficiency of the plant is fundamentally limited by the ratio of the absolute temperatures of the steam at turbine input and output, efficiency improvements require use of higher temperature, and therefore higher pressure, steam. Historically, other working fluids such as mercury have been experimentally used in a mercury vapour turbine power plant, since these can attain higher temperatures than water at lower working pressures. However, poor heat transfer properties and the obvious hazard of toxicity have ruled out mercury as a working fluid.
  Another option is using a supercritical fluid as a working fluid. Supercritical fluids behave similar to gases in some respects and similar to liquids in others. Supercritical water or supercritical carbon dioxide can be heated to much higher temperatures than are achieved in conventional steam cycles thus allowing for higher thermal efficiency. However, these substances need to be kept at high pressures (above the critical pressure) to maintain supercriticality and there are issues with corrosion.